Male infertility can be caused by having a very low sperm count compared with normal ( less than 5 million vs normal 20 million sperm per millilitre of semen). How much semen is produced (ejaculate) is as important as how well the sperm can swim (motility). Whether the majority of sperm appear normal or have abnormal looking heads or tails affects their capacity to swim. Sperm maturity is also important – whether they are mature or predominantly immature. Physical explanations for a low sperm count may include testicles that have not descended properly from the abdomen into the scrotal sac, whether the testicles are small, if a man has been born with a single testicle instead of two or whether the veins in the testicles are abnormally large; such veins can raise the temperature of the testicles causing a decline in sperm count. In addition, previous infection, such as mumps or a past injury can affect how well testicles function and produce sperm, as well as a history of exposure to toxins, drug use, excess alcohol, anabolic steroids, certain medications, or a history of diabetes, thyroid problems, or other hormonal disturbances that affect the development and formation of sperm.